The present invention relates to a heat exchanger mounted on a vehicle such as an automobile. The heat exchanger includes, for example, a radiator for cooling an engine, an air-conditioning condenser, an oil cooler (ATF cooler) for cooling oil of automatic transmission, and an oil cooler for cooling an engine oil.
An automobile includes various heat exchangers. The heat exchangers include heat-exchange tubes through which medium flows, and header pipes connected to the heat-exchange tubes. Each of the header pipes includes communication holes in communication with the heat-exchange tubes. The communication holes become greater in diameter, as a header pipe extends upstream of the medium in the flowing direction. The communication holes become smaller in diameter, as the header pipe extends downstream of the medium. This arrangement uniformly distributes the medium, which flows from the header pipe to the heat-exchange tube (For example, Japanese Patent Application Laid-open No. H9-166368).